HDPE (High-Density Polyethylene) can be a suitable material for making CNC machined prototype parts for the automotive industry. Here are some application examples where HDPE is commonly used:

Fuel System Components:
HDPE is a popular material for CNC machined prototypes of fuel system components, such as fuel tanks, fuel lines, and fuel pump housings.
HDPE's chemical resistance, low permeability, and impact strength make it well-suited for these applications.

Exterior Trim and Body Parts:
HDPE is often used for CNC machined prototypes of exterior automotive trim, such as door handles, side mirrors, and bumper components.
Its relatively low cost, good surface finish, and adequate mechanical properties make it a viable option for these non-structural parts.

Interior Trims and Components:
HDPE can be used for CNC machined prototypes of various interior automotive parts, including air vents, cup holders, and storage compartments.
The material's dimensional stability, impact resistance, and ease of fabrication make it suitable for these types of interior components.

Underbody and Structural Components:
While not as common as other engineering plastics, HDPE can also be used for CNC machined prototypes of certain underbody and structural automotive components, such as skid plates, splash guards, and mounting brackets.
In these applications, HDPE's good strength-to-weight ratio and low cost can be beneficial.

Electrical and Electronic Housings:
HDPE's good electrical insulating properties and resistance to chemicals and environmental factors make it a suitable choice for CNC machined prototypes of electrical and electronic component housings, such as those used in automotive electronics, sensors, and control modules.

The advantages of HDPE for automotive prototyping include its relatively low cost, good mechanical properties, chemical resistance, and ease of machining. However, it's important to note that HDPE may not match the performance of other engineering plastics, such as ABS or Nylon, in certain high-stress or high-temperature automotive applications.